1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, to an image forming apparatus by which an image can be formed on two-page spread, i.e., over both sides of a binding portion.
2. Description of the Related Art
An image forming apparatus having a bookbinding function has been known, for the purpose of binding output images. Specially, in a digital copying machine or a printer which stores images and forms images on sheets of paper allows image formation for bookbinding employing saddle wiring or double-leaved side wiring.
Saddle wiring refers to a method of bookbinding in which sheets of paper are stacked, the sheets are folded at the center, and the sheets are stitched at the center for bookbinding. Image formation for bookbinding employing saddle wiring by a digital copying machine or a printer is performed in the following manner. For example, assume that original documents having the size of A4 fed in transverse direction are to be copied and bound. In the bookbinding, one page has the size of A4. Two images each having the size of A4 are formed on each surface of a sheet having the size of A3 and fed in longitudinal direction. Namely, a total of four images are formed on the sheet, and the sheets are stacked, folded at the center and stitched at the center. Here, transverse feeding direction refers to the feeding direction in which longer side of the document or the sheet is at a right angle to the direction of conveying. The longitudinal feeding direction refers to a direction in which the shorter side of the document or the sheet is at a right angle to the conveying direction. Method of image formation for saddle wiring is disclosed, for example, in U.S. Pat. No. 4,640,611.
The double-leaved side wiring refers to a method of binding in which a sheet is folded into two at the center, the folded sheets are stacked, and open ends of the stacked sheets are stitched. Image formation for bookbinding employing double-leaved side wiring by a digital copying machine or a printer is performed in the following manner. For example, assume that documents having the size of A4 and fed in the transverse direction are to be copied and bound. One page of the book has the size of A4. Two images each having the size of A4 are formed on one surface of a sheet having the size of A3, which sheet is folded into two at the center. Such folded sheets are stacked and stitched at the open end portion of the sheets.
Therefore, in either of the saddle wiring and double-leaved side wiring, one image is formed on the left side and one image is formed on the right side of one surface of the sheet. Therefore, a sheet having such size that allows formation of two-page spread image is desired. For example, when a document having A4 size and fed in the transverse direction is to be copied, it is desirable to use a sheet having the size of A3 and to feed this sheet in the longitudinal direction.
A sheet of an appropriate size is not generally available when a document fed in longitudinal direction is to be copied. When two pages of a document fed in the longitudinal direction are to be copied on a sheet having the size of A3 and fed in the longitudinal direction, using the space of A4 size in the transverse direction as one page, the image should be reduced so that it can be copied within one page or the image should be rotated. However, when the image is reduced to be within one page, the image becomes too small, which is not preferable for the reader. When the image is rotated so that the image in the longitudinal direction is set in the transverse direction, it is not preferable either for the reader, as the original direction is changed.
A method of image formation without reduction of image size or change in direction has been proposed, for example, in Japanese Patent Laying-Open No. 4-149461. In this proposed method, a document having a spread, that is, an image extending over both sides of a binding portion, or an image of a document fed in the longitudinal direction is divided or enlarged and divided, and copied separately onto two pages. In bookbinding copies including such an image divided, or enlarged and divided in the above described manner, it is possible that the image which should be placed on both sides of the binding portion, that is, continuously over two pages is not on both sides of the binding portion but separated in the front and back pages, dependent on the position of the pages on which the image is formed.
U.S. Pat. No. 4,640,611 discloses a method of bookbinding which prevents the divided image from being placed separately on a front and back pages and ensures the divided image to appear on both sides of the binding portion, in two-page spread. According to this method, a portion corresponding to the first page of the sheet is made blank and copying operation is started from an even-numbered page. However, according to this method, it is not possible to form an image on the first page. Further, this method cannot be applied when documents fed in the longitudinal direction and documents fed in the transverse direction are mixed.